There are many kinds of such canned or sealed fluid motor pumps in the world including, for example, the canned motor pump made by Japan Machinery & Instrumentation Co. Ltd. and the canned motor pump disclosed by the U.S. Pat. No. 4,115,038. Much of the literature published in China and foreign countries, while praising the achievement in canned motor pumps, has frequently pointed out the problems needing further improvement.
First, the non-oil-lubricated graphite surfaces generally used in such canned motor pumps as bearings and shaft sleeves are key components, because the wear of bearings will damage their clearance and result in collision of the rotor with the stator that shortens the service life of the pump. Owing to the fact that the bearing of the canned motor pump must be immersed in the fluid medium to be pumped, lubricating oil and lubricating grease can not be applied, so that the lubrication and cooling must be accomplished by the internal circulation of the fluid medium only. However, not all the fluid media that are to be pumped have lubricating properties. To the contrary, many such media possess chemical corrosion properties. Furthermore, the circulating feature of the cooling fluid subjects the bearings to axial thrust; such factors requiring consideration of special bearing materials. At present, graphite and stack welding Co-Cr-W alloy are the generally adopted friction bearing surfaces in all countries. Graphite bearings, however, have small compressive resistance and are strongly affected by the properties and temperature, etc., of the fluid media. In particular, the lubrication of the bearings is affected by the fluid medium itself rather than oil or grease. Therefore, the problem of bearing service life is an urgent problem requiring solution (cf. "Atomic Energy Industry", p399, Atomic Energy Press, China, 1978; "Pipes Arrangement Technology", Japan, 1969, Vol. 11, No. 7).
Secondly, since the bearings of such well-known canned motor pumps are all immersed in the fluid medium, there is a stringent demand on the cleanliness of the medium to be pumped, so that the pump can not be effectively used for delivering fluid media containing tiny suspended particles, limiting the application range of such canned motor pumps ("Power" 1966, V. 110, No. 9).
Thirdly, such canned motor pumps by their design require such accessories as filters, air vent valves and bearing wear-monitors, etc.